Introduction to Linux
Mukesh Pund Scientist,
NISCAIR, New Delhi, India
History
In 1969, a team of developers developed a new
operating system called “Unix” which was written using C
Linus Torvalds, a young man studying computer science at the university of Helsinki developed academic version of Unix which is named as Linux.
Linux is a full UNIX clone .
3
Linux a powerful OS!
Today Linux has joined the desktop market.
On the server side, Linux is well-known as a stable and reliable platform.
Linux provides many applications like:
Databases (MySQL,Postgresql),
Network services(Web Servers,DNS, Proxy, firewall etc)
Software development tools(C, Java, Python,Perl etc.)
Office automation tools
And many more…
Is Linux difficult?
There is excellent and free Internet support and documentation available.
The graphical user interface (GUI) is similar in design to that on any other system
A very powerful command line alternative is also available.
Linux is user friendly.
5
Properties of Linux
It is Open Source
Today, Linux is ready to accept the challenge of a fast-changing world.
Linux is free:
If you want to spend absolutely nothing, you don't even have to pay the price of a CD.
Linux can be downloaded in its entirety
from the Internet completely for free.
Properties of Linux
Linux is portable to any hardware platform.
Linux was made to keep on running.
As with UNIX, a Linux system expects to run without rebooting all the time.
Tasks can be scheduled to run at suitable
times.
7
Properties of Linux
Linux is secure and versatile.
The security model used in Linux is based on the UNIX idea of security which is robust.
It is less prone to virus attacks.
Linux is scalable
Let’s have an overview of frequently used commands in Linux.
Note: Some commands can only be executed by super user (example adduser, shutdown etc).
Commands..
9
Creating a new user
Use the useradd command
Use the passwd command to set password
Try it… logon as root
[root@mukesh]# useradd sdc1 [root@mukesh]# passwd sdc1
Changing password for user sdc1 New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully
[root@mukesh]#
What is a Shell?
Is a program that takes your commands from the keyboard and gives them to the operating system to perform
An interface between the Linux system and the user
Used to call commands and programs
Many available (bsh; csh; bash; etc.)
11
You need help? Add more
In Linux help can be accessed by command man (manual)
Use man <command> to display help for
that command
Linux File System Basics
Linux files are stored in a single rooted,
hierarchical file system
Data files are stored in directories
(folders)
Directories may be nested as deep as needed
Directories
User home directories
Data files
root
13
Some Special File Names
Some file names are special:
/ The root directory (not to be confused with the root user)
. The current directory
.. The parent (previous) directory
~ My home directory
Special Files
/: The root directory where the file system begins.
/boot: This is where the Linux kernel is kept.
/etc: The /etc directory contains the configuration files for the system.
/bin, /usr/bin: These two directories contain most of the programs for the system. The /bin directory has the essential programs that the system requires to operate, while /usr/bin contains
applications for the system's users.
15
Special Files
/sbin, /usr/sbin: The sbin directories contain programs for system administration, mostly for use by the superuser.
/usr: The /usr directory contains a variety of things that support user applications
/lib: The shared libraries (similar to DLLs in that other operating system) are kept here.
/home: /home is where users keep their personal work.
/root: This is the superuser's home directory.
Linux Command Basics
To execute a command, type its name and arguments at the command line
ls -l /etc
Command name
Options (flags)
Arguments
17
Command Options
Command options allow you to control a command to a certain degree
Conventions:
Usually being with a single dash and are a single letter (“ -l ”)
Sometimes have double dashes followed by
a keyword (“ --help ”)
Navigation and Looking Around
pwd - print (display) the working directory
cd < dir> - change the current working directory to dir
ls - list the files in the current working directory
ls -l - list the files in the current working directory in long format
cd ..
19
File and Directory Manipulation
cp <fromfile> <tofile>
Copy from the <fromfile> to the <tofile>
mv <fromfile> <tofile>
Move/rename the <fromfile> to the <tofile>
rm <file>
Remove the file named <file>
mkdir <newdir>
Make a new directory called <newdir>
rmdir <dir>
Remove an (empty) directory
cat > <file>
Create file <file>
Data display from files
cat <file>
Displays contents of the <file>
head -n <fromfile>
Displays n lines from top of the
<fromfile>
tail –n <fromfile>
Displays n lines from bottom of
<fromfile>
21
Standard Files
UNIX concept of “standard files”
standard input (where a command gets its input) - default is the terminal
standard output (where a command writes it output) - default is the terminal
standard error (where a command writes
error messages) - default is the terminal
Redirecting Output
The output of a command may be sent (piped) to a file:
ls -l >output
“>” is used to specify the output file
ls >>output
“>>” is used to append to output
23
Redirecting Input
The input of a command may come (be piped) from a file:
wc <input
“<” is used to specify the input file
Connecting commands with Pipes
The output of one command can become the input of another:
ps -aef| grep postgres | wc -l
The output of the ps command is sent to
grep grep takes input and searches for
“postgres” passing these lines to wc
wc takes this input and counts the lines its output going to the console
“|” is used to separate stages
25
More Commands
who
List who is currently logged on to the system
who am i
Report what user you are logged on as
ps
List your processes on the system
ps -aef
List all the processes on the system
echo “A string to be echoed”
Echo a string (or list of arguments) to the terminal
More Commands
grep - Searches files for one or more pattern arguments. It does plain string, basic regular expression, and extended regular
expression searching
Example: ls –l |grep “mukesh”
ls command display the listing of files in current directory. And grep command searches for “mukesh” file in that listing.
27
More Commands
kill - sends a signal to a process or process group
You can only kill your own processes unless you are root
Example:
[root@mukesh log]# ps –aef
Above command will display result like:
[root@mukesh log]#
UID PID PPID C STIME TTY TIME CMD
root 6715 6692 2 14:34 ttyp0 00:00:00 sleep 10h root 6716 6692 0 14:34 ttyp0 00:00:00 ps -ef
And one can kill the process by following command:
[root@mukesh log]# kill 6715
[1]+ Terminated sleep 10h
More Commands
tar - manipulates archives
An archive is a single file that contains the complete contents of a set of other files; an archive preserves the directory
hierarchy that contained the original files.
tar -zxvf imap-4.7.tar.gz imap-4.7/
imap-4.7/src/
imap-4.7/src/c-client/
imap-4.7/src/c-client/env.h imap-4.7/src/c-client/fs.h
29
Switching Users
su <accountname>
switch user accounts. You will be prompted for a password. When this command completes, you will be logged into the new account. Type exit to
return to the previous account
su
Switch to the root user account. Do not do this lightly
Note: The root user does not need to enter a password when switching users. It may become any user desired. This is part of the power of the root account.
PATH Environment Variable
Controls where commands are found
PATH is a list of directory pathnames separated by colons. For example:
PATH=/bin:/usr/bin:/usr/X11R6/bin:/us r/local/bin:/home/scully/bin
If a command does not contain a slash, the shell tries finding the command in each directory in
PATH. The first match is the command that will
run
31
File and Directory Permissions
Every file or directory
Is owned by someone
Belongs to a group
Has certain access permissions for owner, group, and others
Default permissions determined by umask
File and Directory Permissions
The long version of a listing ( ls -l ) will display the file permissions:
-rwxrwxr-x 1 rvdheij rvdheij 5224 Dec 30 03:22 hello -rw-rw-r-- 1 rvdheij rvdheij 221 Dec 30 03:59 hello.c -rw-rw-r-- 1 rvdheij rvdheij 1514 Dec 30 03:59 hello.s drwxrwxr-x 7 rvdheij rvdheij 1024 Dec 31 14:52 posixuft
Permissions
Owner
Group
33
Interpreting Permissions
-rwxrwxrwx
Other permissions Group permissions Owner permissions
Directory flag (d=directory; l=link)
Changing Permissions
Use the chmod command to change file or directory permissions rwx rwx rwx = 111 111 111 = 777
rw- rw- rw- = 110 110 110 = 666 rwx --- --- = 111 000 000 = 700
chmod 755 file # Owner=rwx Group=r-x Other=r-x chmod 500 file2 # Owner=r-x Group=--- Other=--- chmod 644 file3 # Owner=rw- Group=r-- Other=r--
chmod +x file # Add execute permission to file for all chmod o-r file # Remove read permission for others
chmod a+w file # Add write permission for everyone
35
Changing ownership
chown - change file ownership chown name some_file
chgrp - change a file's group ownership
chgrp new_group some_file
Processes
As with any multitasking operating system, Linux executes multiple, simultaneous processes.
Processes are created in a hierarchical structure whose depth is limited only by the virtual memory available to the virtual machine
A process may control the execution of any of its
descendants by suspending or resuming it, altering its relative priority, or even terminating it
Termination of a process by default causes
termination of all its descendants; termination of the root process causes termination of the session
Linux assigns a process ID (PID) to the process
37
Processes
Foreground
When a command is executed from the prompt and runs to completion at which time the prompt returns is said to run in the foreground
Background
When a command is executed from the prompt with
the token “ & ” at the end of the command line, the
prompt immediately returns while the command
continues is said to run in the background
Process Control Commands
ps - list the processes running on the system
kill - send a signal to one or more
processes (usually to "kill" a process)
39
Process Control Commands
$ ps PID TTY TIME CMD
1280 pts/5 00:00:00 bash 1293 pts/5 00:00:00 xload 1294 pts/5 00:00:00 ps
$ kill -9 1293
[2]+ Terminated xload
Processes
[root@mukesh log]# sleep 10h &
[1] 6718
[root@mukesh log]# ps
UID PID PPID C STIME TTY TIME CMD
root 6718 6692 0 14:49 ttyp0 00:00:00 sleep 10h
& causes process to be run in “background”
Job Number Process ID (ID) Parent Process ID
41
Editors
Several choices available:
vi Standard UNIX editor
xedit X windows text editor
emacs Extensible, Customizable Self- Documenting Display Editor
